i2c: cp2615: Fix 'assignment to __be16' warning

While the preamble field _is_ technically big-endian, its value is always 0x2A2A,
which is the same in either endianness. However, to avoid generating a warning,
we should still call `htons()` explicitly.

Signed-off-by: Bence Csókás <bence98@sch.bme.hu>
Signed-off-by: Wolfram Sang <wsa@kernel.org>